1. Field of the Invention
The present invention generally relates to phase tracking in communication systems that use pilot symbols, including for example, satellite communication systems.
2. Background
For a communication system with a very low signal-to-noise ratio (SNR) such as satellite system, known pilot symbols are inserted periodically in the data frame for the purpose of phase tracking. The pilot symbols are used to correct for Doppler effects, as well as phase noise, that cause phase tracking errors between the incoming carrier signal and a local oscillator within the satellite receiver. Specifically, the pilot symbols are used to estimate the phase of the incoming signal (herein called the phase estimate). For example, the phase estimate can be an estimate of the phase delay introduced in the carrier signal by the Doppler effects.
For example, FIG. 1 illustrates a data steam with pilot symbol slots 102a-d that are inserted in the data stream where each symbol slot can have one or more pilot symbols. The pilot symbols are used to generate the phase estimates, and the phase estimates are used to align the phase of the incoming signal with the phase of the local oscillator in the digital receiver portion of a satellite receiver. This alignment process is known as phase tracking or phase correction. In order to minimize the overhead for pilot symbols, the number of pilot symbols in each slot is kept as small as possible.
Conventional communication systems use phase estimates that are based on one slot of pilot symbols to adjust the phase of the receiver's local oscillator. However, a problem can occur in low SNR communication systems. In these low SNR systems, phase estimates that are based on one slot of pilot symbols may not be sufficient to accommodate the required system performance.
What is needed, therefore, is an apparatus and method of determining the phase estimate of the incoming data signal using multiple pilot symbol slots and then using those phase estimates for the purpose of phase tracking